Just like when wasting meds, don't let another nurse cosign for you if s/he is not really paying attention. And do the same when you are cosigning for them.
Before we administer insulin in our department, we have a policy of verifying with another RN who co-signs that you are giving the proper amount by the proper route, etc. Well, as a new nurse I once used a different type of syringe and drew up 10 times the amount of insulin than what was ordered. I showed it to my (more experienced) co-worker, she glanced over and said "okay" but she obviously wasn't paying attention! Luckily, a few seconds later my brain kicked in to gear and I realized what I had done before I administered the insulin. It was regular insulin ordered IVP, so I would have had a big problem on my hands!
Sometimes like with double-checking blood products we just go through the motions without thinking, especially when we are busy and have our own patients to worry about.